This invention relates to an optical reflection sensor, or a sensor of the type for receiving light reflected from a target object, processing the received light to make a measurement on the target object and outputting the result of this measurement.
Examples of an optical sensor of this type include distance sensors for measuring the distance to a target object and displacement sensors for measuring displacements on a target surface. Prior art displacement sensors are provided with a light source such as a laser diode and a light receiving element such as a PSD (position sensitive device) and a CCD, emitting and receiving light at a specified timing, extracting the focusing position of reflected light on the light receiving surface of the light receiving element according to the timing of light emission and reception and measuring the distance to the target object by using the focusing position by triangulation. Such a displacement sensor may output as its result of measurement the distance to the target object itself or the result of a comparison between the measured distance and a threshold value.
With a sensor of this type, the result of measurement fluctuates delicately, caused by the fluctuations in the analog output from the light receiving element. In the case of a displacement sensor using a PSD as its light receiving element, the distance to a target object is measured on the principle that the relationship between two analog outputs from the PSD changes, depending on the position of incidence of the reflected light. If there is a change in the quantity of reflected light making incidence into the light receiving element, however, the relationship between the two outputs also changes even if reflected light makes incidence at the same position and the measured value fluctuates.
In view of such a problem, it has been known to output an average of a number of measured values obtained while light is emitted and received for a specified number of times such that the accuracy of measurement can be stabilized. It is known that the stability of such averaged data increases as the number of measured values to be averaged is increased. Makers define the accuracy of measurements obtained from averaged data as the repeatability and show to the user the response time, or the time required to obtain a standard level of repeatability. In order to obtain a desired level of accuracy in measurement, the user will adjust the response time or the time to move the target object on the basis of this repeatability.
In general, the repeatability is obtained by test measurements with a white alumina ceramic plate or the like as a target object but since the reflection of light changes, depending on the actual reflectivity, shape and surface roughness of the object, as well as the position of measurement, it cannot easily be expected to obtain the same accuracy of measurement as indicated by the maker.
In order to overcome this problem, it is necessary to keep observing the repeatability of measurements while making these measurements on an actual target object. In order to thus keep observing the repeatability, however, a sensor must be connected to an external device such as a recorder, an oscilloscope and a personal computer. Thus, the process becomes complicated and the operation cost becomes higher. If there is not enough room for setting such external devices near the sensor, furthermore, it is difficult to observe the repeatability in correlation with the condition of the measurement. Even if the repeatability is obtained while a target object is being measured, the measurement may have to be repeated if the reflectivity or the surface roughness of the target object changes, say, because a white target object is replaced by a black object. These are but a few causes of making the processing complicated.